In 1989/92 I was flying a CFM engined B767-300ER and that had, for a while, a disconcerting feature whereby when making an MCP or TOGA selection that required more thrust, sometime only one lever would move forward, sometimes neither and a quick handful was required. A bit hazy now but I believe it was due to the grease on the throttle cable near the hot part of the engine losing its viscosity, eventually the problem was cured by changing the grease, ISTR. We were all aware of the problem and guarded the throttles very carefully.